1. Field of the Invention
The present invention relates to electrolytic fuel cells and assemblies of such cells. More particularly, the invention relates to an electrolytic fuel cell comprising a plurality of manifolds and assemblies of such cells.
2. Background Art
Fuel cells are electrochemical devices which react a hydrogen containing fuel gas with oxygen or an oxygen-containing gas to produce an electric current in an external circuit. Since the process is electrochemical, the level of pollutants released is typically much lower than for combustion based power generation technologies. The conversion efficiency is also higher: 40% to 55% compared 30% to 40% for typical combustion based electric power generation technologies. Fuel cell power generation systems also tend to be quiet and unobtrusive, and can be sited in residential and commercial areas.
Fuel cells have been used in space and other special applications for a number of years, but cost has so far prevented their widespread use for electric utility, industrial and commercial power generation. Certain systems providing up to 11 megawatts output have been demonstrated, and the cost has been greatly reduced through improved design and manufacturing technology and larger scale production. At this point, low enough costs are projected that substantial commercial applications seem possible before the year 2000. These applications are expected to be concentrated in situations in which low pollution is critical, or other attributes such as quiet, unobtrusive operation are necessary. Additional significant cost reductions could make fuel cells attractive to a broader and more cost conscious market.
Two key factors affect the cost per kilowatt of generating capacity. The first is the power output per unit area. As this increases, the cost per kilowatt of output decreases. Unit output is a result of the cell technology and operating variables such as system ambient pressure. The second factor is the material, labor and purchased component cost per unit area. Clearly, this cost directly affects the cost per kilowatt. In typical present day fuel cell designs the cost of the active electrochemical elements is often less than half the total expense for the particular fuel cell. The majority of the cost is for support, gas channeling and similar inactive elements.
Phosphoric acid, molten carbonate and solid oxide electrolyte fuel cells are at various stages of development for utility scale power generation. Phosphoric acid and molten carbonate designs are generally stacks of planar cells, and solid oxide designs include both planar and tubular configurations.
The active portion of a molten carbonate fuel cell is a layer of ceramic powder, saturated by molten carbonate salts, sandwiched between a porous metallic nickel anode and a porous nickel oxide cathode. The cell operating temperature typically is about 650.degree. C. Fuel gas containing hydrogen is passed over the anode, and air containing oxygen is passed over the cathode. The gases react indirectly with each other through the electrolyte, causing an electrical potential to develop between the cathode and the anode. The cell can therefore supply power to an external circuit connecting the cathode and the anode. Typical electrical potential of such molten carbonate cells can be from 0.6 to 0.9 volts.
Multiple cells are combined by stacking individual cells in electrical series. The voltage generated by the cell stack is the sum of the individual cell voltages. Additional components must be added to the active components to build a stack. For example, separator plates are placed between the cells to separate the anode gases of one cell from the cathode gases of the adjacent cell. These are typically stainless steel with a protective nickel cladding layer on the anode side. Seals and flow conduits must be provided to channel fuel gas to the anodes and air to the cathodes, without mixing of the gas streams. Corrugated metal current collectors may be added to space the anode and cathode away from the separator plates, and provide passages to facilitate gas flow over the electrodes. The current collectors must provide a good electrical connection between the anodes and cathodes and the separator plates, and withstand the compressive loads which clamp the stack together. Additional components such as end plates, clamping mechanisms, gas supply and return manifolds, power takeoffs and insulation are required for a complete stack assembly.
The size of molten carbonate fuel cells has been scaled up over the years to achieve manufacturing and installation economies. Larger components require less handling labor per square foot. Dead area at the edge seals decreases as a percentage of total material area, increasing the proportion of material which is actively utilized. Further, the cost of non-repeat parts such as manifolds, end plates, axial load systems, and containment vessels tends to increase more slowly than active area and power output as the cell size increases. System plumbing and interconnection costs decrease per unit of output as the generating capacity of individual stacks increases. Increases in cell size have been paced by the ability to manufacture wide sheets of metal and active cell material with acceptable quality. Widths of up to 3 feet and areas of 4 to 10 square feet are now the state-of-the-art.
While such increases of cell sizes provide some advantages, larger cell size can also pose significant disadvantages. As cell size increases, the mass flow of gas across the electrodes increases in rough proportion to the flow path length. This in turn requires higher flow passages which add to the stack height without adding to power output per unit area.
Larger cells also increase cell complexity and material content. The separator plates and current collectors must bridge the gas flow passages while carrying compressive load and current, and must limit the contact pressure applied to the electrodes. Two or three formed heat and corrosion resistant alloy metal sheets are typically required per cell in current systems to form a structure which carries out these functions. These sheet metal assemblies are generally the largest cost element in the stack.
Furthermore, as cell complexity increases cell performance can decline. Multiple layers of sheet metal in pressure contact introduce resistive potential losses, and the electrode areas blocked by the current collectors introduce concentration gradient potential losses.
Still further, temperature differences across the cell structure can increase as cell size increases. The changing reaction and gas temperature conditions across the cells induce temperature differences and consequent stresses. The large cell size and the thinness of the components inhibit heat flow in the plane of the cell which would tend to even the temperatures out.
Small cells, in contrast, have comparatively short flow paths, and do not require current collectors. The anode and cathode porosity, augmented by dimples or grooves in the surfaces in contact with the separator plates, provides sufficient flow area. Thus in the case of small cells, bulky separator plate flow conduits are not required. This results in comparatively thin cell assemblies which have high volumetric power density and low material content. Further, the small size results in moderate thermal gradients.